State of the Art Relating to Multi-Phase Materials
First of all, in the prior art, documents relating to the formation of two-phase materials made of gold and tungsten are known. For example, the article of SVEN HAMANN ET AL may be mentioned: “Synthesis of Au microwires by selective oxidation of Au—W thin-film composition spreads”, published in SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS, vol. 14, no. 1, 1 Feb. 2013 (2013 Feb. 1), page 015003, XP055250552, ISSN: 1468-6996, DOi: 10.1088/1468-6996/14/1/015003. This publication relates to the growth of microwires formed by matrix consisting of a non-homogeneous mixture of gold and tungsten. This document never mentions the formation of a homogeneous alloy, which is not the object of the works. FIG. 1b of this prior art document further shows the dispersion in concentration of the two elements on the substrate scale: there is no composition homogeneity across the whole sample. This document never invokes the formation of a homogeneous alloy, and on the contrary shows that it leads to the preparation of samples having pure areas of gold and pure areas of Tungsten. On page 4, the last paragraph of this document, the authors admit only that at the initial state (just after deposit), the samples “the diffraction pattern in the as-deposition state (not shown) indicate the presence of pure at least one of Au and W”. It's clearly the evidence that these samples are not single-phase since they contain at least these two pure phases. Their method therefore does not lead to a single-phase and homogeneous gold-tungsten alloy.
Another publication, the article of OSSI et al. may be cited: “Model of formation in ion-mixed binary alloys with positive training heats of formation”, published in THE JOURNAL OF THE LESS-COMMON METALS, ESEVIER-SEQUOIA S. A. LAUSANNE, CH, vol. 160, no. 2, 1 May 1990 (1990 May 1), pages 351-362, XP024073636, ISSN: 0022-5088, DOi: 10.1016/0022-5088(90)90393-X.
This scientific article describes experimental works related in another publication mentioned in note 19. It is the article “Ion Beam Mixing of Selected Binary Metal Systems with Large Positive Heats of Formation” of W. HILLER, M. BUCHGEISTER, P. EITNER, K. KOPITZKI, V. LILIENTHAL and E. PEINER, institute fiir Strahlen- and Kernphysik from the University of Bonn, Nussallee 14-16, D-5300 Bonn 1 (F.R.G). The referenced document exposes the manufacturing mode of the samples which are the object of this OSSI et al. article.
It consists of alternating deposits of thin layers of pure Au and W produced by vacuum evaporation (electron gun). The samples are then irradiated with a beam of ions Kr+ at 400 keV. The ion mixing is the definition of the diffusion effects of the two metals (Au and W) under the effect of the irradiation. FIG. 1 of reference 19 shows X-ray diffraction spectra before and after irradiation.
The non-irradiated state clearly shows the presence of diffraction peaks relating to Au and W, which is an evidence that the sample is not single phase. Moreover, it cannot be the case since it is to be composed of pure layer alternations of each metal. The figure of this article shows that it does not lead to the presence of a single-phase and homogeneous alloy. In addition, the arguments of the authors will be in this direction.
Relating to the preparation of materials comprising a gold phase and a tungsten phase, Japanese patent JP 2012 212662 is also known, relating to the organic electronics in which it is reported (00141) a deposit of a material of Au—W (80:20 by mass) by sputtering, leading as for the two previous documents to a heterogeneous structure. This document never mentions the production of a single-phase homogeneous alloy.
State of the Art Relating to the Materials of the “White Gold” Type
It has already been proposed in Swiss patent CH-684 616 a nickel-free grey gold alloy with good deformability, which typically includes in this case essentially between 15% and 17% by weight of palladium, 3 to 5% of magnesium and 5 to 7% by weight of copper.
Patent application EP2427582 is also known, which relates to a nickel and copper-free white gold alloy having a hardness which is particularly suitable for watchmakers and jewellers. This alloy according to the prior art consists of (in weight %): more than 75% of gold, more than 18% to less than 24% of palladium, more than 1% to less than 6% of at least one element selected from Mn, Hf, Nb, Pt, Ta, V, Zn and Zr, and if necessary, not more than 0.5% of at least one element selected from Si, Ga and Ti and optionally not more than 0.2% of at least one element selected from Ru, Ir and Re.
A white gold composition is also known from U.S. Pat. No. 6,863,746, consisting essentially of copper, silver, zinc and manganese, and further comprising low amounts of tin, cobalt, silicon/copper and boron/copper. More particularly, the white gold composition of the present invention describes a white gold composition consisting essentially of about 36% to about 57% of copper, about 10% of silver, about 18.2% to about 24.2% of zinc, about 14% to about 28.9% of manganese and the remainder being further comprised of about 1% of tin, from about 0.025% to about 0.03% of cobalt, about 0.52% of silicon/copper, and about 0.2% of boron/copper.
International patent application WO2014108848 describes an alloy composition for the production of gold alloys, more particularly white gold alloys comprising rhodium (Rh) used as a white layer component and capable of reducing the release of nickel upon the insertion of rhodium (Rh) in nickel-containing alloys. Patent application EP2045343 describes another nickel-free white gold alloy containing gallium Ga as a whitening agent.
The patent EP2450461 describes a nickel and copper-free grey gold alloy having a hardness which is particularly suitable for watchmakers and jewellers. This alloy consists of (in weight %):
more than 75% of Au;
from more than 18% to less than 24% of palladium;
from more than 1% to less than 6% of Zr;
optionally, from more than 1% to less than 6% of at least one element selected from Mn, Hf, Nb, Pt, Ta, V and Zn;
optionally, at most 0.5% of at least one element selected from Si, Ga and Ti; and
optionally, at most 0.2% of at least one element selected from Ru, Ir and Re.
Patent EP1010768 describes a nickel-free white gold alloy, in particular for the lost-wax casting technique, comprising, by weight, 75-76% of gold and 12-14% of palladium, 7-11% of Cu, between 1 and 4% of In, between 0.2 and 0.4% of Ga, the remainder being formed by a proportion of between 0.01 and 4% of at least one of the elements Ir, Re, Zn, Nb, Si, Ta, Ti.
Disadvantages
Prior art solutions have different disadvantages. These alloys have physical characteristics (density, hardness, ductility, corrosion resistance, tarnish resistance and yellowing resistance) which are not fully satisfactory. Many of these solutions use palladium, which results in good whiteness. However, it is a very expensive metal and the prices of which fluctuate greatly, leading to too soft alloys in the absence of other metals; thus, the solutions using this metal are not well-suited.
The addition of copper proposed in most prior art solutions certainly makes it possible to harden the alloys but its thermal inertia results in difficulties in casting a part and causes, during heat treatment, non-controllable hardening and cracking risk. Furthermore, the copper has an oxidation risk. For most alloys of the prior art containing Pd and/or Cu, it is necessary, to obtain the colour and brightness of the desired metal, to carry out a galvanic deposit of rhodium. The thickness of this coating (a few hundred nanometers) remains sensitive to the friction and the colour of the substrate reappears punctually, which does not allow the production of gold objects intended to last.
In order to avoid the need for a rhodium-plating, a gold alloy must ensure, according to the standard ASTM Method D1925, a value YI: D1925<19 (YI: “yellowness index”), considered as “a good white” or “premium”, and integrated with the Grade 1 (see Proceedings of Santa Fe Symposium 2005, pp. 103-120). The value YI may be transposed into the CIELab system, CIE being the abbreviation of International Lighting Commission and La*b* the three coordinate axes, the L axis measuring the white-black component (black=0 and white=100), the a* axis measuring the red-green component (red=positive values, green=negative values) and the b* axis measuring the yellow-blue component (yellow=positive values, blue=negative values) (Cf. standard ISO 7724 established by the International Commission of the Illumination). The colours of the gold alloys are set in the tri-chromatic space according to the standard ISO 8654. A value YI<19 corresponds in a first approximation to [−2≤a*≤2; b*<10].